1. Field of the Invention
This invention relates to apparatus for the play-back of information from an optical video disk, and more particularly to information play-back apparatus in which the size of the spot produced by the light beam illuminating the optical video disk is varied according to the pattern of an information recorded portion.
2. Description of the Prior Art
As is well known, there have been developed a technique wherein optical recording is made at a size of about 1 micron on a disk plate or disk sheet, video signals or digital information being recorded under the condition under which the phases or amplitudes in such recorded portions (called "pits") are made different from those in other portions of what is called the video disk and wherein the signals or information are played back from the video disk.
In this case, since the inner track on the disk which is closer to the center of the disk and the outer track closer to the fringe or outer rim of the disk have different lengths, the form or slope of the pits to be recorded on the respective tracks are made different. That is, the pits on the outer side are more elongate along the track than the pits on the inner side so that the same number of pits can be provided on each track. This is based on the fact that the speed of revolution of the disk is constant and so, the number of pits per revolution should be constant.
Heretofore, the read-out of the signals has been carried out by illuminating the pits of the different shapes by a converging light beam having a constant spot size at all times. Accordingly, the signals obtained from the tracks on the outer side and on the inner side would naturally differ.
More specifically, in prior art play-back apparatus for reproducing information stored on an optical video disk, in playing back video and audio signals recorded within a region of 5 cm-15 cm in radius on the disk having a diameter of about 30 cm, the spot size on the disk of a He-Ne laser beam employed in the play-back apparatus has always been maintained constant. It is therefore impossible with such apparatus to obtain from tracks on both the outer periphery and the inner periphery of the disk good signals, i.e., signals having little deformation and high S/N (signal-to-noise) ratio. Usually, the spot size is set so as to be suitable for the tracks on the outer periphery side of the disk. At the inner periphery, accordingly, the S/N ratio lowers and the deformation of the signal waves increases, so that moire or beat effects appear in a played-back image. This is attributed to the fact that the lengths of the information pits differ by a factor of three between the outer and inner peripheries of the disk, and that 2-3 information pits are simultaneously illuminated at the inner periphery when an optimum spot size for the outer periphery is used. On the other hand, when the spot size is made most suitable for the tracks at the inner periphery, the spot size is too small to completely illuminate the pits in the track at the outer periphery. Accordingly, the modulation depth of the signals lowers sharply, and the S/N ratio at the outer periphery becomes low.
This will be more specifically explained with reference to FIGS. 1(a)-1(c).
FIG. 1(a) shows the situation in which the same play-back light beam 2 is projected onto pits 11 on an inner track 1a and pits 12 on an outer track 1b. FIG. 1(b) indicates that, when the intensity distribution of the illumination beam is selected to achieve on the inner track 1a a play-back signal of high S/N ratio and high output power, noises (higher harmonics) arise in the signal derived from the outer track 1b, as depicted by a curve 1b". Further, FIG. 1(c) indicates that, when an illumination beam intensity distribution appropriate for the outer track 1b is used, the output becomes low on the inner track 1a as depicted by a curve 1a". That is, in either case, a good play-back signal is not obtained. In FIGS. 1(b) and 1(c), curves 1a' and 1b' represent the play-back outputs of the inner track 1a and the outer track 1b, respectively, in the case where the intensity distributions of the illumination beam are selected to be optimum for each track.